I am rebuilding a 1981 Porsche 924 and currently rebuilding the brakes. This car had set for 15yrs so the Master cylinder reservoir had a nasty black filmish gunk inside as well as bad fluid. Before reusing this reservoir it was necessary to clean it.

I went through the normal cleaning with alcohol and a couple of hydrocarbon solvents but the film persisted. To complicate matters, one reservoir was accessible through the filler cap but the back reservoir only had access through the bottom cylinder feed hole. It was virtually impossible to get anything in there to scrub the side walls and tank bottom.

As I thought about, it dawned on me that somewhere I had read about using steel pieces in solvent and then sealing and shaking the tank to get an abrasive action. I settle on using steel BB gun shot balls. They are small, fairly heavy and easily go inside the bank tank whole.

I poured a fair quantity in the tank, put in some isopropyl alcohol and some hot water with a good detergent. I sealed the tank and shook the tank back and forth for a long time. This loosened the gunk and it came out with the liquid and shot once the plugs were removed. I repeated this 3 or 4 times and the resulting solvent finally game out clear. It was easy enough to remove the BB's by holding over a small can and shaking them out through the MC feed holes. The few remaining BB's were then removed by sticking a small magnet down the feed hole and capturing the BB's as they rolled by. For BB's I got some at Walmart for $1.46 a tube which was more than enough. They are reusable so they can be rinsed, dried and saved.

There is still some dark shading to the tank where the film was but after much review I concluded that this was staining from the long time exposure to dirty brake fluid. The tank should be useful for a long time to come.

I recently used about a tablespoon of baking soda and poured in a little white vinegar. Pour slowly as it will foam out quickly. I then let is sit for a couple hours and then flush it out with water until it is clean. Make sure it get to the rear portion of the reservoir as that is where all my accumulation was.
Gary_________________78 924 Special Edition